Just about every pilot’s operating handbook and airplane flying manual makes mention of the same property: “The fuselage is of a conventional semi monocoque construction…” And that’s the last mention of semi monocoque construction anywhere in the book. I’m willing to bet that your training manuals make little to no mention of it either.

Airplane Fuselages, the Old Fashioned Way

The Vickers Warwick's geodesic skeleton

Much like the foundation of a house, the fuselage is the foundation of an airplane. It is the central attachment point for the wings, tail and engines. As such, it needs to be strong enough to support the loads imposed by the weight of the airplane in maneuvering flight.

Locked into the paradigm of their time, aviation pioneers constructed their fuselage much the same as a building or bridge. They used sturdy materials to construct heavy internal skeletons that would bear the full load of flight. It was not until 1913 that a Swiss man challenged the traditional wisdom of fuselage design.

Monocoque Construction

Notice the lack of any internal bracing in the Deperdussin monocoque racer

Eugene Ruchonnet thought outside of the box. Rather than mounting panels onto a solid load-bearing skeleton, Ruchonnet’s idea was to let the airplane’s skin carry the load. He did this by forming the fuselage out of multiple layers of wood. The layers were glued together with their grains running in different directions to strengthen the skin so much that no internal skeleton was needed! He called the new technique monocoque construction, or single-shell construction.

Monocoque construction works fine with tiny airplanes, but there is a point at which the weight of the monocoque fuselage is greater than the weight of an internal skeleton. Furthermore, monocoque construction is unforgiving of even the slightest structural failure: think of how easily an aluminum can is crushed once a single dimple forms in it’s skin and you’ll see the inherent problem.

Semi Monocoque Construction

Bulkheads and supporting beams (stringers) reinforce stressed skin in semi monocoque airplanes

In order to build bigger and stronger airplanes, a hybrid of the two construction techniques was put forward and remains in use today. The idea behind semi monocoque construction is quite simple. Instead of building a full internal skeleton, aircraft designers chose to build a partial skeleton to reinforce the skin in critical areas. A semi monocoque airplane’s skin supports much of the load, with some internal bracing and bulkheads in place to maintain structural integrity. This design works surprisingly well, and remains in place on most modern aircraft from single engine pistons to the brand new Boeing 787 Dreamliner.

If you look closely at commercial aircraft, you might notice something that looks a bit like an extra engine. Consider the CRJ-900 for example. It clearly has only two engines, but take a look at the tail. It has an additional jetpipe which surely resembles a third engine.

Learn all about how the APU provides redundancy in my guest article at AskaCFI.com